Automated pipetting stations are designed to transfer multiple measured aliquots of liquid from one multi-well plate to another multi-well plate. Such pipetting stations comprise a pipetting head capable of engaging with multiple disposable pipette tips. For convenience of handling, the pipette tips are supplied pre-loaded in a pipette rack.
FIG. 1 is a cross-sectional view showing an example of a typical pipette tip 10 suitable for use with an automated pipetting station. Pipette tip 10 has a neck portion 12 and a tapered portion 14. Neck portion 12 engages with a respective one of the barrels (not shown) of the pipetting head of the pipetting station. The pipetting head and its constituent barrels and the pipetting station are not shown in FIG. 1 but are described below with reference to FIG. 7A. Neck portion 12 is hollow and has an internal bore 13 whose diameter tapers from a maximum at the proximal end of pipette tip 10 and then remains substantially constant over the remaining length of the neck portion. The terms proximal and distal denote position relative to the pipetting head of the pipetting station. The constant-diameter portion of the bore 13 of neck portion 12 has a contoured inner surface 16 that provides positive engagement with the barrel of the pipetting head. The contouring is not shown to simplify the drawing. Tapered portion 14 extends axially from the neck portion 12. Tapered portion 14 is hollow and has a tapered bore 15. Tapered portion 14 and bore 15 have respective diameters that gradually reduce with increased distance from neck portion 12. At least part of neck portion 12 is surrounded by a collar 18. Collar 18 is larger in diameter than the neck portion 12 itself. Collar 18 has a distal surface 20 orthogonal to the longitudinal axis of pipette tip 10.
Pipette racks are supplied in pre-configured stacks of, for example, 20 pipette racks. Each pipette rack is pre-loaded with a defined number, e.g., 384, of pipette tips similar to pipette tip 10. To reduce the height of the stack of pipette racks, and, hence, the space needed to store the pipette tips, the pipette racks are configured to nest when they are stacked, and the pipette tips in each pipette rack nest in the pipette tips in the pipette rack below. The pipette racks nest in the sense that a lower part of an upper pipette rack accommodates an upper part of a lower pipette rack on which the upper pipette rack is stacked. The pipette tips nest in the sense that part of the tapered portion 14 of a pipette tip 10 in the upper pipette rack is located inside the neck portion 12 of a pipette tip 10 in the corresponding location in the lower pipette rack.
At the beginning of a protocol in which pipette tips are used, a stack of nested pipette racks is loaded into a rack stacker at the pipetting station. A robotic arm takes a pipette rack from the bottom of the stack and aligns the pipette rack with the pipetting head. The pipetting head moves downwards to engage with the pipette tips in the pipette rack, moves upwards to remove the pipette tips from the rack and then performs a liquid transfer process using the pipette tips. The pipetting head then moves downwards to return the used pipette tips to the rack. The robotic arm then discards the pipette rack holding the used pipette tips.
As noted above, it is advantageous to stack the pipette racks with the pipette racks and the pipette tips in a nesting arrangement to conserve packaging and storage space. However, when a conventional nestable, stackable pipette rack is stacked on another, similar pipette rack with the pipette tips in a nesting arrangement, the pipette tips tend to bind or cling to one another, resulting in one or more of the pipette tips being ejected from its rack. Ejected pipette tips are problematical because they can cause the equipment handling the pipette racks to jam.
When conventional nestable, stackable pipette racks are used, and there are not enough pipette racks remaining in the stack of pipette racks loaded in the rack stacker to perform the next protocol, the remainder of the stack of unused pipette racks is removed from the rack stacker and is discarded. A new stack of pipette racks is then loaded into the rack stacker and the next protocol is run. This wastage occurs because such conventional pipette racks are not configured to allow additional pipette racks to be added to a partial stack of pipette racks. Moreover, such conventional pipette racks cannot be re-stacked in another rack stacker after the pipette tips in them have been used. Consequently, such conventional pipette racks holding used pipette tips have to be discarded individually.
One solution to the problems of mechanical binding and electrostatic cling is to provide an intermediate tip support plate between the pipette racks to prevent the pipette tips from nesting too closely, if at all. The intermediate tip support plate increases the spacing between the pipette racks so that close nesting between the pipette tips does not occur. This allows the pipette racks to be stacked without mechanical binding or electrostatic attraction between the pipette tips that can lead to problems when such pipette racks are stacked. However, the intermediate tip support plate undesirably increases the stacking pitch, which decreases the number of pipette racks that can be stacked within a rack stacker of a given size and increases the storage space required to store a given number of pipette racks.
Accordingly, what is needed is a nestable, stackable pipette rack that allows maximum nesting of the pipette tips, that allows pipette racks to be added to a depleted stack of pipette racks prior to performing a protocol, and that allows pipette racks to be automatically re-stacked after the pipette tips in them have been used.